The present invention relates to atrium-controlled heart pacemakers. In such pacemakers, generation of a ventricular stimulation pulse occurs after a delay time following the appearance of an atrial signal.
In heart pacemakers of this type, the stimulation frequency can vary within a relatively wide range of, for example, 50 through 150 pulses per minute. An atrium-controlled pacemaker contains a limiting stage which defines the highest pulse rate. The heart pacemaker thus does not always function completely reliably, particularly when an atrial signal appears within certain time limits relative to a stimulation pulse in the ventricle.
It is an object of the present invention to provide an atrium-controlled heart pacemaker which guarantees reliable operation independently of the chronological appearance of an atrial signal with respect to a ventricular stimulation pulse.
The above object is achieved in accordance with the principles of the present invention by a heart pacemaker having a switching stage controlled dependent on the appearance of atrial signals, the switching stage switching the pacemaker to ventricular inhibited (VVI) mode for a predetermined plurality of stimulation pulses or natural ventricular events upon the appearance of at least one atrial signal occurring before a selected time, the selected time being derived from the difference between a smallest synchronous signal and the AV delay.
In a pacemaker constructed in accordance with the principles of the present invention, reliable operation is further guaranteed upon the appearance of atrial signals within an interval which is smaller than the smallest interval defined by the highest allowable stimulation pulse rate.
When an atrial signal appears within the smallest programmed interval between two ventricular stimulation pulses which has been reduced by the AV delay time, the pacemaker can be switched for generating either a low pulse rate or a high pulse rate in the VVI mode. A predetermined number of ventricular stimulation pulses will be emitted in any event, the predetermined number being, for example, between one and sixteen pulses. The pulse rate supplied in the VVI mode, accordingly, is between the base rate and the highest synchronous pulse rate of the pacemaker.
In the case, wherein an atrial signal appears within the interference interval, the pacemaker can be switched to a prescribed low pulse rate which can be one of the programmed base rates. Because this operating mode is intended to protect against external interference, the pulse rate should accordingly be at least approximately equal to the base rate.
An additional feature of operation with the highest synchronous pulse rate is the capability of protecting the heart pacemaker against tachycardia induced by the pacemaker. This is guarded against because the high VVI pulse rate is programmed to be slightly lower than the highest synchronous pulse rate. Because tachycardia induced by the heart pacemaker results in the heart pacemaker stimulating the heart with the highest synchronous pulse rate, the selected difference between the highest synchronous pulse rate and the high VVI pulse rate makes tachycardia induced by the heart pacemaker impossible.